A problem associate with hydrocarbon conversion processes, especially catalytic processes, at elevated temperatures is the deposition of carbon in the reactor. Carbon deposition or coking can result on the reactor walls and/or on the surface of the catalyst. Coke deposition on reactor walls reduces heat transfer, can result in an increase in pressure drop and ultimately a decrease in overall reaction efficiency. Similarly, carbon deposition on catalysts reduces efficiency resulting in decreased conversion and/or selectivity.
Carbon deposition on catalysts and on reactor walls is generally a greater problem during processing of heavier hydrocarbon feeds. The relative involatility of the molecules makes condensation in catalyst pores or on reactor walls more likely and, once condensed, coke forming reactions such as polymerisation and dehydrogenation are accelerated. Residue-containing feedstocks, in particular, have a high tendency towards carbon deposition. Again, this is due to both physical and chemical effects. Feeds containing vacuum residue will normally consist of some very large molecules called asphaltenes which can boil at up to 1000.degree. C. The involatility of such molecules accelerates coking. In addition, the residues contain relatively large amounts of aromatic molecules. These are low in hydrogen/carbon ratio and can act as nucleation sites for coking via polymerisation reactions.
When carbon deposition reaches intolerable levels, the process must be stopped and the carbon removed. Typically, carbon removal or decoking is carried out by gasification in the presence of steam or by burning in the presence of air or a combination of both. In each case external heat is required. Alternatively, high pressure water jets may be used to remove the carbon layers from the walls of the reactor. Once the carbon has been removed, the process may be re-started. The obvious disadvantage with this method of treatment is the time lost in stopping and re-starting the process.
U.S. Pat. No. 4,917,787 discloses a method intended to overcome the aforementioned problems through the development of on-line decoking wherein the cracking process is periodically stopped and the hydrocarbon feed replaced with a steam-containing feed for a period of time sufficient to reduce the carbon deposits. The hydrocarbon feed is turned off whilst maintaining a methane-hydrogen/oxygen feed to a burner. Additional stem is added to the reactor and the decoking is carried out by the action of steam at temperatures in excess of 1200.degree. C. which gasifies the carbon in the pyrolysis region of the reactor.